A new study in mice is the latest to show why it’s important for scientists to include both male and female test subjects in their research. The study suggests that a common diabetes drug—which is now being explored as a way to repair the brain—may only help boost cognition in young children and adult women.
The drug is called metformin, and since the 1950s, it’s been one of the most common medications used by people with type 2 diabetes to help manage and lower their blood sugar. More recently, though, evidence has started to show that metformin might also be able to help the brain repair itself faster from injury, as well as stave off the sort of neurological damage that leads to cognitive impairment and dementia.
According to study author Cindi Morshead, a stem cell biologist and chair of the Division of Anatomy at the University of Toronto, metformin might do this by encouraging the growth of existing stem cells in the brain. And if that’s true, then metformin could be an invaluable treatment for all sorts of neurological injuries and disorders.
“When we think about repairing the injured or diseased brain, one of the promising approaches is taking advantage of ‘resident stem cells’ that are present in the brain,” she told Gizmodo in an email. “These stem cells are able to generate all the different cell types that comprise the brain, which holds promise for having them replace lost cells.”
Her team had already found in experiments that metformin could repair the brain in newborn mice who had suffered a stroke, which cuts off blood circulation to parts of the brain and can cause otherwise irreparable brain damage. That success encouraged them to experiment with using metformin on stroke-damaged mice of varying ages.
But once they included juvenile teens and adults, the effects became much less apparent. On average, adult mice treated with metformin didn’t perform any better on maze tests of their memory than control mice did. As they dug deeper into the initial results, though, they noticed a clear sex difference—adult females actually did do better on metformin.
They then theorized the drug’s neural effects were being influenced by either or both of the sex hormones found in many animals, including mice and humans: estradiol (a form of estrogen) or testosterone. They performed various experiments to test the theory, such as removing the ovaries of females (cutting off their supply of estrogen) and castrating male mice (cutting off their supply of testosterone).
Ultimately, they concluded that estrogen seems to enable the brain-related effects of metformin on brain stem cells in adult females, while testosterone inhibits its activity in adult males. In both scenarios, Morshead said, these sex hormones are likely “altering the ‘microenvironment’ that the stem cells are in and this, in turn, alters the stem cell behavior.”
Newborn mice of either sex experience a short burst of estrogen soon after birth, while mice going through puberty experience rising levels of both sex hormones, possibly explaining why metformin works and doesn’t work in these two groups, respectively.
The study’s findings, published Wednesday in Science Advances, are especially prescient. Earlier this May, scientist Rebecca Shansky of Northeastern University in Boston wrote about the ongoing need to stamp out sex bias in medical research, starting with lab animals, and the harm it can cause. The lack of women in clinical trials of the sleep aid Ambien, for instance, made us miss that the drug has more potent and potentially dangerous effects on women, necessitating a lower recommended dose for them.
“This study highlights how critical it is to consider sex as a variable in research,” Morshead said. “If we had combined our male and female mice for this study, we would have concluded that metformin is not effective in promoting recovery.”
The sex and age-related effects of metformin on the brain, if they apply to people as well as mice, might also explain why it’s been hard to find consistent evidence for the drug’s brain-boosting ability in studies so far, according to Morshead.
“Absolutely, sex could play a role in the outcomes of studies,” she said.
Members of Morshead’s team, which include doctors at the Hospital for Sick Children in Toronto, are currently running a small trial testing metformin for children who have suffered brain injuries. And there are similar clinical trials going on elsewhere. These and future studies, Morshead said, will have to make sure both men and women are being given the consideration they need.
“Human studies are complicated and challenging on a number of levels, and considering sex adds a layer of complexity, but the [work on metformin] underscores the importance of evaluating sex-dependent outcomes,” she said. “The impact of considering sex is too important to ignore.”
In addition to human research, Morshead and her team plan to continue using mice to study how and why these sex-related differences form, whether these differences can be seen with other types of brain injury beside stroke, and if there are ways to ensure that everyone can someday benefit from metformin.